Optical fiber amplifiers and preamplifiers are employed in a variety of technologies, including telecommunications fields. A fiber amplifier typically includes a gain fiber, the core of which includes rare-earth dopant ions, such as erbium or neodymium. One or more laser diodes (or other pumping means) are coupled to the fiber to provide a pump signal to the fiber core. When optically pumped, the fiber exhibits gain over a wavelength region characteristic of the rare-earth dopant. The amplifier gain is related to the amount of pump power coupled to the gain fiber as well as to the length of the fiber. Also, the output power of the amplifier may influence the frequency response of the amplifier to signal modulation.
Both single mode and multimode fibers have been utilized to implement optical fiber amplifiers. A multimode fiber amplifier can propagate and amplify multiple modes (e.g., hundreds or thousands of modes) whereas a single mode fiber amplifier amplifies a single mode. In a multimode mode fiber, modes are a set of guided electromagnetic waves that travel unchanged in an optical fiber except for gain or loss. The modes thus can be described as solutions of Maxwell's equation for electromagnetic waves propagating in the bound core medium of the fiber.
The different transverse modes also travel at different speeds through the multimode fiber, which is referred to as modal dispersion and results in multimode distortion. Multimode distortion is a mechanism that occurs in multimode fibers in which the signal is spread in time because the velocity of propagation of the optical signal is not the same for all modes. For example, those rays which undergo the fewest number of reflections while propagating through the core will traverse the length of the fiber sooner than those rays which undergo a greater number of reflections. As a result of multimode distortion, the useful communication bandwidth of multimode fibers is limited. Additionally, amplified spontaneous emissions can occur for both single mode and multimode fiber amplifiers, which can add noise and reduce gain of the fiber amplifier. In view of these and limitations inherent in multimode fiber amplifiers, there has been a general reluctance to develop and utilize multimode fiber amplifiers in certain commercial technologies; consequently, potentially useful applications have been overlooked or neglected.